Method of recovering plutonium values from aqueous solutions by carrier precipitation



Unite Satesv Patent Fice METHOD OF RECOVERING PLUTONIUM VALUES FROMAQUEOUS SOLUTIONS BY CARRIER PRECIPITATION Ralph A. James, Santa Monica,and Stanley G. Thompson, Concord, Califi, assignors to the United Statesof America as represented by the United States Atomic Energy CommissionNo Drawing. Application December 19, 1951 Serial No. 262,476

6 Claims. (Cl. 2314.5)

drazine, it was found, greatly impairs the carrier precipi tation ofplutonium, for instance, on bismuth phosphate, one of the carrierspreferred used, and the efficiency of the plutonium recovery process isthus greatly reduced. This proved particularly true when such solutions,from which the plutonium was to be recovered, contained great quantitiesof uranium which is always the case in dissolver solutions. The chemismof the deleterious reaction or reactions of the hydrazine is not knownwith certainty; studies showed that it is quite involved and that mostlikely a number of factors or reactions are responsible for the highplutonium losses in the presence of hydrazine. One essential factor,however, seems to be that hydrazine reduces plutonium to the trivalentstate and holds it in this valence and that plutonium (III) is not verywell carriable on bismuth phosphate, at least not as completelycarriable as is the tetravalent plutonium. This, too, is particularlypronounced when uranium is present in relatively great amounts.

Frequently the plutonium in the solutions, prior to carrier preciptation, is secured in its tetravalent state by adding formic acid to thedissolver solution. Formic acid reacts with nitric acid and formsnitrous acid which reduces plutonium (VI) to plutonium (IV) and/or holdsany tetravalent plutonium present in this state. Also this nitricacid-formic acid reaction was found to be inhibited by the presence ofhydrazine.

Carrier precipitation processes have been carried out in which thehydrazine had been destroyed prior to processing by sodium nitrite. Butthe results of those experiments were highly erratic, and the plutoniumlosses were too high in most instances; the satisfactory resultsobtained in a few experiments were not reproducible.

It is an object of this invention to provide a process for the recoveryof plutonium values from aqueous solutions by carrier precipitationwhich has a high degree of efiiciency.

It is another object of this invention to provide a process forconditioning nitric acid plutonium solutions for carrier precipitationwhereby detrimental side products are eliminated.

It is still another object of this invention to provide a process forconditioning plutonium-containing nitric acid solutions for carrierprecipitation whereby practically complete plutonium recovery is alwaysobtained.

All of the above-described disadvantages are overcome and the objects ofthis invention are accomplished by treating the nitric acid plutoniumsolution to be processed with an oxidizing agent. The oxidizing agentobviously destroys the hydrazine but at the same time oxidizes at leastsome plutonium (IV) to plutonium (VI). Plutonium (VI) is not carriable,for instance, on bismuth phosphate so that a subsequent reducing stepfor the conversion of the hexavalent plutonium to the plutonium (IV),the so-called prereduction, is often advisable, although not alwaysnecessary, to make the solution ready for carrier precipitation withbismuth phosphate.

Many substances are suitable as oxidizing agents for the process of thisinvention; however, potassium dichromate, sodium dichromate andpotassium permanganate are preferred. The concentration in which theoxidizing agent has to be present in the solution is dependent uponthecomposition of the solution; however, a concentration ranging between0.001 N and 0.03 N was found suitable in all cases tested. The optimalconcentration range is that between 0.02 and 0.03 N. It is advisable touse elevated temperature for the oxidation in order to accomplishcomplete destruction of the hydrazine. For instance, heating to from toC. for about one hour proved sufiicient, a temperature of 75 C.representing the best conditions.

There are also a great many substances that are suitable as theprereducing agent to be added after the preoxidation step. Sodiumnitrite or a mixture of oxalic acid and a manganous ion-containingsubstance, e.g., manganous nitrate, is the preferred reducing agent forsecuring the plutonium in the tetravalent state. The concentration ofthe reducing agent may vary widely, but a concentration of about 0.1 Mfor the sodium nitrite and of about 0.03 M and 0.01 M for the oxalicacid and the manganous ion, respectively, have proved best.Hydroxylamine in a concentration of about 0.1 M has also givensatisfactory results.

It was found advantageous to carry out the prereducing step in thepresence of sulfuric acid in order to avoid reduction of the plutoniumto the trivalent state. A concentration of about 0.5 M sulfuric acid issufficient for this purpose. The function of the sulfuric acid isprobably a complexing reaction with the tetravalent plutonium.

Elevated temperatures are also advisable for the prereducing step, andheating for one hour at about 75 C. represents the preferred condition.

In the following a few examples of the process of this invention aregiven for the purpose of illustrating the improvement obtained therebywithout the intention to have the invention limited to the details giventherein.

Example I To a 20% aqueous uranyl nitrate hexahydrate solutlOncontaining 25 mg. of plutonium per liter of solution and being 1 N innitric acid and 0.001 M in hydrazine were added 2.5 mg. of bismuth perml. of solution in the form of bismuth subnitrate; the solution washeated for four hours at 75 C. Thereafter, a 75% phosphoric acid wasgradually added within two hours in a quantity to obtain a concentrationof 0.6 M. The solution was then digested by holding it at 75 C. for onehour. The precipitate formed was removed by centrifugation and analyzedfor its plutonium content. It was found that 40.3% of the plutoniumoriginally present had remained in the solution.

A parallel experiment was carried out using the same solutions andconditions except that potassium permanganate was added to the solutionprior to the four-hour heating in a quantity to obtain a concentrationof 0.003 N. The subsequent precipitation, carried out as describedabove, yielded a 99% removal of plutonium by carrying on bismuthphosphate.

Example II A simultated dissolver solution was made up by dissolvinguranium metal in nitric acid, adding, during disthe solution so as toobtain a concentration of 22.5% uranyl nitrate hexahyrate. Thereafter,there were added sulfuric acid to obtain a concentration of 1.1 N andtetravalent plutionium to obtain a concentration of 25 mg./liter ofsolution. The nitric acid concentration of the solution was 0.25 N.These concentrations correspond to those present in actual radioactivedissolver solutions. Nine IO-ml. aliquots were used for nine parallel p4 hydrazine-containing nitric acid solution, consisting in adding anoxidizing agent selected from the group consisting of alkali metaldichromate and potassium permanganate to said solution; digesting saidsolution with the oxidizing agent at a temperature of between 75 and 100C.; adding sulfuric acid to said solution; adding also at about75 C. areducing agent selected from the group consisting of sodium nitrite,oxalic acid plus manganous nitrate and hydroxylamine to saidsolution'whereby the modified within the scope of the appended claims.

What is claimed is: 1. A process of improving plutonium precipitation onbismuth phosphate from an aqueous plutoniumand tests. Hydrazine wasincorporated in seven of these aliplutonium is secured in thetetravalent state; and then quots, in each case in a quantity to obtaina concentra precipitating a bismuth phosphate carrier in said solution.tion of 0.001 M, while the two other aliquots were used 2. The process.of claim 1 wherein the oxidizing agent without hydrazine. The solutionswere heated whereis added in a quantity that it is present in thesolution after all of the plutonium in the hydrazine-containing in aconcentration of from 0.001 to 0.03 N.

samples was found to be present in the trivalent state. 3. The processof claim 2 wherein the concentration Precipitation tests were carriedout on each of the of the oxidizing agent ranges between 0.02 to 0.03 N.IO-ml'. aliquots. The precipitation conditions used were 4. The processof claim 1 wherein the reducing agent practically identical with thosedescribed in Example I. is sodium nitrite and wherein it is present inthe solution Various kinds of pretreatment, however, preoxidation in aconcentrationof about 0.1 M. and/ or prereduction, were applied prior toprecipitation. 5. The process of claim 1 'wherein the reducing agent Theconditions of pretreatment used in these nine exisa combination ofoxalic acid and manganous nitrate periments are shown in the table belowtogether with which are present in respective concentrations of aboutthe results obtained thereby. 0.03 M and 0.01 M.

' Percent Percent Exp. N2H4, Preox-Idatlon Prereduction Treatment Pu PuN o. M Treatment Loss Material Balance KMnO4 (0.01M),1 B20204 0.03 M),.n(II) 4.3 105.7

hr. 75 0. (0.01 M 1 1112, 75 0. do do 4.5 104.3 01 K1Cr1O1 (0.01M), -.do3.3 103.8

1 hr.. 75 C. o 2.1 105.7 Nnnp dn 37.1 101.6 n N 40.3 106.1 do 42.1 101.6 0 K2CI'207(0.011\1),1 11,0104 (0.03 M), Mn(II) 1.5 99.5

hr. 75 C. (0.01 M 1 hr., 75 0. None dn 1. 6 100. 5

It will be obvious from this table that with no pre- 6. The process ofclaim 1 wherein the reducing agent 7 oxidation, with. or withoutprereduction, the plutonium 40 is hydroxylamine and wherein it ispresent in a concen' loss amounted to approximately 40%. The plutoniumtration of about 0.1 M.

losses were reduced to from 2.1 to 4.5% by the preoxida 7 tion treatmentaccording to this invention, potassium di- References Clted m the fileof this patent chromate showing slightly superior results to those ob-UNITED STATES PATENTS tamed with potassium permanganate. In experiments8 2,785,951 Thompson et a1 Man 19, 1957 and 9 for which hydrazine-freesolutions were used, the 2,799,553 Thompson et a1? July 16, 1957plutonium losses were as low as 1.5 and 1.6 with and 2,311,415. SeaborgOct 29, 1957 without preoxidatron, respectively.

It will be understood that this invention is not to be "OTHER REFERENCESlimited to the details given herein but that it may be Harvey et al.:The Chemistry of Plutonium, Journal of the Chemical Society, pp.1010-1011 1947).

Seaborg et al.: The Transuranium Elements, pp. 59 and 60 (1949); pub. byMcGraw-Hill, New York, as IV-14B of the National Nuclear Energy Series.

1. A PROCESS OF IMPROVED PLUTONIUM PRECIPATION ON BISMUTH PHOSPHATE FROMAN AQUEOUS PLUTONIUM AND HYDRAZIENR-CONTAINING NITRIC ACID SOULATION,CONSISTING INADDING AN OXIDIZING AGENT SELECTED FROM THE GROUPCONSISTING OF ALKALI METAL DICHROMATE AND POTASSIUM PERMANAGANATE TOSAID SOULATION; DIGESTING SAID SOULATION WITH THE OXISIZING AGENT AT ATEMPERATURE OF BETWEEN 75 AND 100*C.; ADDING SULFRIC ACID TO SAIDSOULATION; ADDING ALSO AT ABOUT 75*C. A REDUCING AGENT SELECTED FROM THEGROUP CONSISTING OF SODIUM NITRITE, OXALIC ACID PLUS MANGANEOUS NITRATEAND HYDROXYLAMINE TO SAID SOULATION WHEREBY THE PLUTONIUM IS SECURED INTHE TETRAVALENT STATE; AND THEN PRECEIPATING A BISMUTH PHOSPHATE CARRIERIN SAID SOULATION